1. Field
Disclosed herein are treads for tires, in particular for tires intended to be used on snow-covered ground. Also disclosed are tires for touring vehicles equipped with such treads.
2. Description of Related Art
It is known to provide a tire tread with a sculpted design including a plurality of cut-outs of different sizes, notably with the aim of evacuating water present in rainy weather between the road surface and the tread. It is thus possible to ensure adhesion of the tire to said road surface that is satisfactory from a safety point of view. By “cut-out” is meant a hollow space of a tread delimited by walls of rubber mixture or any other material forming the tread, these walls facing each other and leading onto the tread, where they form facing edges.
According to the width of these cut-outs, a distinction is made between grooves (at least 2 mm wide) and incisions (less than 2 mm wide); thanks to this small width the latter offer the advantage of closing at least partially on coming into contact with the road surface, i.e. at least in part the facing walls delimiting these incisions are in contact one against the other. All the cut-outs form edges on the surface of the tread, this surface (called the “rolling surface”) being in contact with the road surface; these edges are very important for improving rolling performance, notably on a snow-covered road surface or a wet road surface in rainy weather.
Winter tires rolling on a snow-covered road surface may exhibit the following disadvantage: the cuts forming the sculpted design fill with snow and retain this snow from one coming into contact to the next. If snow fills the cuts, it is no longer rubber-snow adhesion that determines the behaviour of the tire, but snow-snow adhesion. Obviously, snow will tend to fill narrow cuts faster.
To address this disadvantage, it has been proposed to provide each incision with areas widened relative to the width of this incision, in order to increase the storage capacity of the incision. The document EP 0847878 seeks to improve the traction of a tire for snow-covered ground and to this end proposes a tire tread including a cut-out leading onto the rolling surface and having a depth less than the thickness of the tread, this cut-out forming two facing edges on the rolling surface of the tire when new, these edges delimiting a plurality of narrow portions and at least one wide portion, these wide and narrow portions being disposed alternately and extending the full depth of the cut-out. The wide portions have a width decreasing progressively toward the interior of the tread.
The document EP 1190871 describes a solution making it possible to ensure non-closure of a cut-out when the tire is subjected to acceleration or braking. This solution consists in locally reducing the width of each incision by forming a sort of protuberances. The document describes a tread including at least one cut-out leading onto the rolling surface and having a depth less than the thickness of the tread, this cut-out forming two facing edges on the rolling surface of the tire, these edges delimiting a plurality of narrow portions and a plurality of wide portions, the wide and narrow portions being disposed alternately, the narrow portions extending the full depth of the cut-out, the wide portions extending into the depth of the tread to at least 30% of the depth of the cut-outs. Wide portions are formed between the protuberances and narrow portions are formed at the location of the protuberances.
The sculptural designs proposed in the cited references have a number of drawbacks. In particular, the structure of the bottom of the cut-out entails the risk of cracks arising and thereby compromising the durability of the tire.
The document JP-7276923 describes a tread comprising blocks provided with incisions comprising narrow parts and wide parts disposed alternately.